1. Field of the Invention
The present invention relates in general to a piezoelectric and/or electrostrictive actuator (hereinafter referred to as "P/E actuator"), and more particularly to a uni-morph, bi-morph or other types of P/E actuators or actuator units having a plurality of piezoelectric and/or electrostrictive elements (hereinafter referred to as "P/E elements") formed on a ceramic substrate thereof. The present invention is concerned with such P/E actuators or actuator units which undergo flexural displacement upon actuation of the P/E elements and which are used as ink pumps of an ink jet print head.
2. Discussion of the Related Art
As a means for raising a pressure in a pressure chamber formed within a ceramic substrate of an actuator unit, there is recently known a P/E element formed on a wall partially defining the pressure chamber so as to change a volume of the pressure chamber due to displacement of the P/E element. Such a piezoelectric and/or electrostrictive actuator unit may be used for each of a plurality of ink pumps of a print head of an ink jet printer, for example. To effect printing by the print head, an ink material is supplied to the pressure chamber to fill the same, and the level of ink pressure within the pressure chamber is raised due to displacement of the P/E element, so that a droplet of the ink is jetted through a nozzle communicating with the pressure chamber.
The assignee of the present invention proposed one example of ink jet print head, as disclosed in U.S. patent application Ser. No. 08/066,193 which employs the above-described P/E actuator unit. More specifically described, the proposed actuator unit has an actuator portion comprising: a ceramic substrate consisting of a plurality of ceramic green sheets which are superposed on each other and sintered into an integral laminar structure having pressure chambers therein; and a plurality of P/E elements each of which is formed by a film-forming method on an outer surface of a wall which partially defines the corresponding pressure chamber of the ceramic substrate. This actuator portion is bonded to a suitable ink nozzle member, thereby to provide an integral structure of the proposed actuator unit. Generally, an ink jet print head uses many actuator units arranged side by side.
Described in detail referring to FIGS. 5 and 6, an actuator portion generally indicated at 12 used in an ink jet print head comprises a ceramic substrate 14 obtained by integrally sintering a laminar structure of ceramic green sheets superposed on each other and a plurality of film-like P/E elements 16 formed on an outer surface of the ceramic substrate 14 by a film-forming method. The ceramic substrate 14 has a plurality of pressure chambers 18 formed therein which are aligned with the respective P/E elements 16. The thus constructed actuator portion 12 is bonded to an ink nozzle member indicated at 20 which includes a plurality of nozzles 22, ink discharge holes 24 and an ink supply channel 26. In operation, the ink material which is fed into the ink supply channel 26 is supplied to the pressure chambers 18 of the actuator portion 12 through respective through-holes 28. Upon actuation of the P/E elements 16, the upper walls of the pressure chambers 18 are deformed, thereby to raise the pressure of the ink within the pressure chambers 18. Then, the ink material within the pressure chambers 18 is jetted outwards from the appropriate nozzles 22 through the corresponding through-holes 29 of the ceramic substrate 14 and the corresponding ink discharge holes 24 of the ink nozzle member 20.
In an attempt to provide an ink jet print head which is capable of performing highly accurate and high-quality printing, a relatively large number of pressure chambers 18 are formed within the substrate 14 of the actuator portion 12 and arranged at a relatively small pitch, and a correspondingly large number of nozzles 22 are formed for effecting highly accurate and high-quality printing.
The ink jet print head employing the actuator portion 12 constructed as described above in each actuator unit thereof, however, has suffered from a problem in terms of the operating characteristics of the P/E elements 16. That is, among the plurality of P/E elements 16 which are formed in a row on the substrate 14 so as to be aligned with the respective pressure chambers 18, two outermost elements 16 which are located at the opposite ends of the row are inferior in the operating characteristics to the other or adjacent inner elements 16 interposed therebetween.
More specifically described, in the actuator unit 12 constructed as described above, the amount of flexural deformation of each P/E element 16 (the amount of flexural deformation of the upper wall of each pressure chamber 18) was measured by applying a suitable voltage to each of the elements 16. The result of the measurement showed that the amounts of flexural deformation of the above-indicated two outermost piezoelectric/electrostrictive elements 16 are smaller by about 30% than those of the other, inner elements 16 interposed therebetween. Thus, the two outermost elements do not undergo sufficiently large amounts of flexural deformation as compared with the inner elements. A variation or fluctuation in the amount of the flexural deformation or inconsistent operating characteristics of the P/E elements 16 of the actuator portion 12 undesirably causes inconsistency or fluctuation in the ink-jetting capability of the print head and deterioration of the quality of printed images produced by the print head, when the above-constructed actuator units are used as ink pumps for the ink jet print head.
As a result of a further study by the inventors of the present invention, there was proposed a P/E actuator as disclosed in U.S. Pat. No. 5,376,856 in an attempt to solve the above-described problem, wherein the actuator portion of each actuator unit has two dummy cavities or enclosed spaces within the substrate formed adjacent to respective two outermost pressure chambers which are located at opposite ends of a row of the pressure chambers, such that the two dummy cavities sandwich the row of pressure chambers. In this arrangement, the rigidity of the substrate at the two outermost pressure chambers located at opposite ends of the row is effectively reduced due to the existence of the dummy cavities, whereby the two outermost chambers are more likely to be deformed, and therefore, the two outermost P/E elements corresponding to the two outermost pressure chambers exhibit improved operating characteristics which are substantially equal to those of the other inner elements interposed between the two outermost elements.
However, in the above actuator portion which is provided with the above-described dummy cavities formed adjacent to the two outermost pressure chambers, the size and configuration of the dummy cavities must be suitably selected for enabling the two outermost P/E elements corresponding to the two outermost pressure chambers to exhibit the operating characteristics substantially equal to those of the inner elements sandwiched by and between the two outermost elements. Usually, the selection or adjustment of the size and configuration of the dummy cavities must be effected by trial-and-error depending upon the operating characteristics of the P/E elements of each actuator portion. As a result, there arises a problem that it takes a lot of time and cost to initially design the actuator portion and subsequently modify the design of the actuator portion. In particular, where windows which give the pressure chambers and dummy cavities are formed by punching off the corresponding portions of a green sheet which constitutes a part of the ceramic substrate, various kinds of metal molds used for punching operation must be prepared in order to obtain the dummy cavities having desired sizes and configurations for permitting the two outermost P/E elements to exhibit improved operating characteristics. The preparation of the metal molds requires considerable cost and time, undesirably pushing up the manufacturing cost of the actuator portion of the actuator unit.